1. Technical Field
The present invention relates to memory devices, and more particularly, to flash memory devices and program methods thereof.
2. Discussion of the Related Art
Recently, mobile devices have become smaller, thinner and lighter allowing for easy portability. In addition, a single semiconductor chip used in a smart card or an RFID tag must be able to implement a variety of functions. Therefore, system-on-chip (SoC) technology capable of implementing a central processing unit (CPU), a digital signal processor (DSP), a memory, etc. on one chip is being employed in semiconductor chips that are used in portable terminals or smart cards. A memory device mounted on the SoC of a portable terminal includes dynamic random access memories (DRAMs), static random access memories (SRAMs) and also nonvolatile memories such as flash memories. A volatile memory device has a high read/write operation speed but loses stored data when an external power supply is interrupted. On the contrary, a nonvolatile memory device can retain stored data even, when an external power supply is interrupted. Therefore, the nonvolatile memory device is used to store data (e.g., boot codes) that must be retained even without the external power supply. In general, a NOR flash memory with a high operation speed is widely used as a flash memory mounted on the SoC. The NOR flash memory has a high operation speed but is undesirable for high integration.
The demand for high integration is increasing in the fields of mobile devices or smart cards. For the smart cards, extensive research is being conducted to satisfy the need for high-speed write/read operations in one chip and the need for high-capacity storage. One of the attempts to satisfy the need for high speed/capacity is to install a NAND flash memory in a smart card chip. Hereinafter, such a NAND flash memory is referred to as an embedded NAND flash memory.
However, a NAND flash memory and a NOR flash memory are different in terms of the methods used for their programming. F-N tunneling is performed to inject electrons into a floating gate in the NAND flash memory, whereas hot electron injection is performed to inject electrons into a floating gate in the NOR flash memory. Therefore, the NAND flash memory and the NOR flash memory need different bias conditions. When the NAND flash memory and the NOR flash memory are mounted on one chip, multiple high-voltage generators are used to satisfy the different bias conditions.